Not Applicable
1. Technical Field
The present invention relates to a dental tool assembly having a head that imparts oscillatory motion to a desired dental treatment device coupled to the assembly. More particularly, the present invention relates to a drive mechanism for a dental tool assembly, the drive mechanism having a rotating drive shaft that engages a first end of a driven shaft to rotate the driven shaft in an oscillatory manner. The present invention also relates to a bearing assembly for a dental tool drive mechanism, the bearing assembly supports the drive shaft for rotary motion using a bushing shape to reduce friction while also aligning the drive shaft. A dental tool is coupled to a second end of the driven shaft and is thereby rotationally oscillated.
2. Description of the Related Art
Dental tool assemblies, such as prophy angles and drills, which impart an oscillatory rotary motion to a dental treatment device coupled thereto are known in the art. In particular, such assemblies typically have a driving mechanism comprising a drive shaft with a rotation axis that is perpendicular to the rotation axis of a driven shaft to which the dental treatment device is coupled. The drive shaft of prior art driving mechanisms has an element positioned eccentric to its rotation axis and extending towards the driven shaft to engage a slot in the driven shaft. Rotation of the drive shaft thus imparts an oscillatory rotation to the driven shaft.
For example, U.S. Pat. No. 1,711,846 to Heilbom shows a dental filing device having a drive shaft perpendicularly oriented with respect to a file holder. A crank pin, mounted on a crank disc on an end of the drive shaft adjacent the file holder, engages within a bore in the file holder. The crank pin is positioned on the crank disc eccentric to the rotation axis of the drive shaft. Thus, rotation of the drive shaft rotates the eccentrically positioned stud, thereby causing the file holder to rotate in an oscillatory manner.
Similarly, the dental instrument in U.S. Pat. No. 2,135,933 to Blair has a rotary drive shaft with an eccentrically positioned stud that engages within a slot of a piston to which a massage tip is coupled. Rotation of the drive shaft causes oscillatory rotation of the massage tip. Another massage tool that imparts oscillatory motion to a head spindle to which a massage cup or brush is coupled is shown in U.S. Pat. No. 4,534,733 to Seigneur in et al. In the Seigneur in Patent, the stud that engages the head spindle is mounted eccentric to the rotation axis of the drive shaft, but is inclined to extend across the rotation axis. The portion of the stud that is aligned with the rotation axis of the drive shaft is also aligned with the rotation axis of the head spindle. The dental tool shown in U.S. Pat. No. 4,460,341 to Nakanishi also has a guide pin mounted eccentric to the rotation axis of a drive shaft and engaging within a slot of a driven shaft to which a dental treatment device is coupled.
In all of the above-described dental tool assemblies, a stud or pin extends into a slot to drive the element to which the dental treatment device is coupled. Because the treatment device typically must be driven at very high speeds (e.g., the recommended speed of a standard prophy angle at approximately 6,000 rotations per minute), there is a risk of the stud or pin breaking off during use. Moreover, manufacturing of the drive shaft and driven shaft is complicated by the necessity of forming a stud and a slot that are shaped for ready, secure engagement such that rotation of the drive shaft causes oscillatory rotation of the driven shaft.
Additionally, some of the drive shafts of the above-described patents also impart reciprocatory axial motion to the driven shaft along the longitudinal shaft of the driven shaft. When such axial motion is not desired, the driven shaft should be locked with respect to the housing in which the drive shaft and driven shaft are positioned, and thus locked with respect to the rotation axis of the drive shaft. Typically, such locking is accomplished by locking the driven element with respect to the housing such as by inter-engagement of stepped portions and/or flanges. However, such locking imparts substantial stresses against the housing and driven shaft.
Another drawback of the above-described devices is that they are typically formed from metal and are reusable. The sterilization process necessary in order to reuse the device is typically costly and time consuming. It therefore has been desirable to provide disposable dental tool assemblies that are used only once and therefore need not be sterilized. Such tools typically are made from plastic.
Because plastics are generally not as strong as metals, the driving mechanism used in the above-described devices cannot be used because of the inherent weakness of the stud. Therefore, the driving mechanisms of disposable dental tools typically have interengaging gears, such as shown in U.S. Patent No. 5,571,012 to Witherby et al. Because gears are used, the same reciprocatory rotary motion provided by the non-disposable tools cannot be achieved. However, such oscillating movement is desired for a number of reasons. The back and forth reciprocating motion provided by non-disposable dental tool assemblies permits greater speeds to be used and greater pressure to be applied than rotary type devices that do not oscillate, and also may massage the gums of the patient. Additionally, oscillatory movement generates less heat than a full rotational action. Moreover, the risks of hitting undercuts, cutting or tearing soft tissue, and splattering of agents applied by the treatment tool are reduced if not substantially eliminated.
Another problem faced by any drive mechanism is how to provide support and alignment for the moving parts. There are many types of bearings that may be used for this purpose. Due to the size, construction methods, and materials used in prophy angles, journal or sleeve style bearing configurations are generally used.
An example of such a bearing is shown in U.S. Pat. No. 5,340,310 issued to Bifulk on Aug. 23, 1994. As shown in FIGS. 3, 4 and 5 thereof, a housing having a cylindrical bore with fingers spaced from and extending axially parallel to the bore are provided for receiving a bushing having a square cross-section such that the bushing abuts turned in portions of the fingers. The fingers are flexible such that they bend outwardly allowing the bushing to be pushed past the turned in portions yet bend back to capture the bushing therein.
Another example of a drive mechanism is shown in U.S. Pat. No. 5,931,672 issued to R. Postal on Aug. 3, 1999. The mechanism disclosed therein uses a series of flanges and a latch that includes a position retaining surface that contacts the outer periphery of at least one of the flanges. The extreme outer edge of the flanges provide axial alignment for the drive shaft.
In use, this arrangement is susceptible to heat build-up on the various bearing surfaces. This problem is compounded by the difficulty in lubricating the bearing surfaces during assembly of the components. Therefore it is desirable to have a bearing surface that is less susceptible to heat build-up and is easily lubricated during assembly. It is also desirable to have a bearing arrangement that is less complex to assemble. The present invention addresses these desires for improving the bearing arrangement in prophy angle drive mechanisms.
It is therefore an object of the present invention to provide a disposable dental tool assembly having a driving mechanism that imparts oscillatory rotary motion to a dental treatment device mounted on the assembly and to achieve this with a structure that can be economically, and reliably implemented in plastic to allow for disposability and the attendant avoidance of the spreading of infection.
It is a related object of the present invention to provide a driving mechanism having a drive shaft and a driven shaft each having driving surfaces shaped to engage each other and ride along each other such that rotation of the drive shaft causes oscillatory rotation of the driven shaft.
It is a further object of the present invention to provide a dental tool assembly having driving and driven elements that are stabilized with respect to each other against relative movement in a given direction.
It is another object of the present invention to provide a dental tool assembly having a drive shaft that is coupled to a driven element such that the drive shaft imparts only oscillatory motion to the driven element without also imparting axial motion to the driven element.
These and other objects of the present invention are accomplished in accordance with the principles of the present invention by providing a dental tool assembly having a rotating drive shaft that engages a driven shaft to impart oscillatory rotary motion to the driven shaft. The drive shaft and driven shaft are positioned transverse to each other. The drive shaft has a driving surface at its distal end that is shaped to engage a driven surface on a side of the driven shaft adjacent the drive shaft. Because of the manner in which the distal end is shaped, a stud or guide pin, such as used in the prior art, is no longer needed. Specifically, the driving surface is a cutaway, curved portion of an enlarged end of the drive shaft, and the driven surface is a cut-away side portion of the driven shaft. The cut-away portions of each shaft are shaped to interengage with substantially no play there between such that they are in continuous contact during rotation of the driving shaft. Because of the shapes of the cut-away portions, rotation of the driving shaft causes oscillatory rotation of the driven shaft.
The drive shaft and driven shaft are positioned within a housing. In order to prevent relative movement of the shafts with respect to the housing, a plurality of locking mechanisms are provided. First, the drive shaft is provided with a longitudinally extending pin aligned with the rotation axis of the drive shaft. The driven shaft is provided with a slot through which the pin is passed. The slot is shaped so that oscillatory rotation of the driven shaft is not inhibited by the pin, yet axial movement of the driven shaft along its rotation axis is prevented. Another locking mechanism for the drive shaft is provided in the form of at least one flange extending radially from the drive shaft and engaging a radially inwardly extending flange on the inner surface"" of the housing. The driven shaft is provided with a rearwardly positioned pin that fits within a bore in the housing to lock the driven shaft in the desired position for oscillation.
In the alternative, a bearing assembly acting as both a thrust bearing and a retainer for holding the drive shaft in place is characterized in that it includes a bushing having an inwardly facing frustoconical surface acting in concert with a journal attached to the drive shaft and having a contact surface in contact with said frustoconical surface, said bushing also having a snap-fit arrangement on a peripheral surface thereof.